Dry cleaning is a major industry throughout the world. In the United States alone, there are more than forty thousand dry cleaners (many of these have multiple locations). The dry cleaning industry is an essential industry in the present economy. Many articles of clothing (and other items) must be dry cleaned in order to remain clean by removal of body fats and oils, and presentable by preventing shrinking and discoloring.
The most widely used dry cleaning solvent until now has been perchloroethylene (PERC). There are numerous disadvantages to PERC including inherent toxicity and odor.
Another problem in this field is that different fabrics require different handling in the presently used systems in order to prevent damage to the fabrics during the dry cleaning process.
Prior art dry cleaning processes include the use of various solvents with appropriate machinery to accomplish the cleaning. As mentioned earlier, the solvent most widely used has been PERC. PERC has the advantage of being an excellent cleaning solvent, but the disadvantage of being a major health and environmental hazard, i.e., it has been linked to numerous forms of cancer and it is very destructive to ground water and aquatic life. In some areas PERC is prohibited due to these disadvantages. Additionally, in the past, other solvents such as petroleum-based solvents or hydrocarbons have been tried and used. These various solvents are less aggressive than PERC, but are still classified as volatile organic compounds (VOC's). As such, such compounds are regulated and permitted by most air districts.
The dry cleaning industry has long depended on petroleum-based solvents and the well-known chlorinated hydrocarbons, perchlorethylene and trichlorethylene, for use in the cleaning of fabrics and articles of clothing. Since the 1940's, PERC was praised as being a synthetic compound that is non-flammable and has great degreasing and cleaning qualities ideal for the dry cleaning industry. Beginning in the 1970's, PERC was found to cause liver cancer in animals. This was an alarming discovery, as dry cleaning waste was placed in landfills and dumpsters at that time, from which it leached into soil and ground water.
Environmental Protection Agency regulations gradually were tightened, culminating in a law that took effect in 1996 that required all dry cleaners to have "dry to dry" cycles, meaning that fabrics and articles of clothing go into the machine dry and come out dry. These required "closed loop" systems can recapture almost all PERC, liquid or vapor. The process "cycle" involves placing fabrics or articles of clothing into a specially designed washing machine that can hold 15 to 150 pounds of fabrics or articles of clothing that are visible through a circular window. Prior to being placed into the machine, the fabrics or articles of clothing are checked and treated by local hand spotting for stains. If the fabric is unusual or known to be troublesome, the label is checked to verify that the manufacturer has deemed the item safe for dry cleaning. If not, the stain may be permanent. As an example, a sugar stain may not be seen, but once it is run through the dry cleaning process, it oxidizes and turns brown. If the stain is grease related, water won't help, but solvent will as it solubilizes grease. In fact, the principle reason for dry cleaning certain clothes (which should not be washed in a regular washing machine) is to remove the build up of body oils (known as fatty acids) because they too oxidize and produce rancid nasty smells.
The grease and fatty acids which build up in the solvent is removed by filtration and by distillation of the solvent. In other words, the dirty solvent is boiled and all vapors are condensed through a condensation coil back to a liquid. The liquid recovered is comprised of both solvent and water and the liquid is then passed through a separator in order to separate the two non-miscible liquids. The water may originate from the natural humidity of the ambient air exposed to the textiles prior to cleaning. Another source of moisture may be materials used during pre-spotting.
Before textiles are removed from the machine, the washer becomes a dryer. Hot air is blown through the compartment but, instead of being vented outside, the air stream goes through a condenser that condenses the vapors to liquid. The liquid then passes through a separator to decant off the water from the solvent and return the solvent for reuse.
If the water is not separated from the solvent, the water will carry over into an associated storage tank and due to its density will settle on the bottom of the tank. If the level of water is sufficient it will be picked up by the pump system and may be pumped onto the articles being cleaned which would result in damaging the articles.
If the water sits on the base tank for a sufficient amount of time, bacteria will begin to grow which will result in a very bad odor that will transfer to the articles being cleaned. The hydrocarbon solvent is a feed stock for bacteria and may quickly contributed to the growth of bacteria. The interface level between the lighter density solvent and the more dense water causes an interface level between the water and solvent. The polar solvent soluble contaminants in this interface level may include fatty acids, food, perspiration, and general body odor. The extended settling can quickly result in the growth of bacteria and the end result of odor.
It is therefore very critical for professional dry cleaning to control the presence of water in such a way as to not damage the articles being cleaned or cause odors that would result in customer dissatisfaction.
One of the criteria in the selection of a proper water/solvent separation system is the difference in the density or specific gravity of the solvent and water. The density or specific gravity of PERC (the most commonly used solvent) is 1.619, as compared to water which is 1.0. The next most commonly used type of solvent is the petroleum based type or hydrocarbon solvent whose specific gravity ranges between 0.754 and 0.820 with the most common hydrocarbon solvent (DF-2000) being 0.77. The greater the difference in specific gravity between the water and the solvent, the easier it is to separate the two. Gravity separators have been designed and are used when the solvent is either denser or less dense than the water and the density difference between the phases is greater than 0.03.
While systems have been developed to separate water and solvents with a specific gravity vastly departed from that of water (1.0), no efforts have been made to separate water and solvents with a specific gravity closer to 1.0.